An electric arc furnace is an apparatus which is designed for melting a metal charge, supplied in the form of bulk ferrous scrap, in order to obtain a bath of molten steel. This liquid metal may be refined in the arc furnace itself, in order to convert it into a steel possessing specified properties, or this refinement may not be carried out. The electric arc furnace is primarily a tool for melting ferrous scrap, and this is becoming increasingly noticeable in modern steel plants.
An arc furnace is equipped with three graphite electrodes. Each of the electrodes is equipped with a separate raising and lowering device which, during the melting of a charge of ferrous scrap, enables the electrode to be, at any moment, at the appropriate distance from the scrap. The arcs are struck between each of the electrodes and the charge to be melted. The current which flows in the electrodes is a three-phase alternating current, which is substantially balanced and has a magnitude of the order of tens of thousands of amperes. In comparison with the impedance of the arc, the power supply circuit presents an impedance which is not negligible, whereby the arc voltage represents only about 70 to 90% of the voltage at the transformer terminals. The impedance of the power supply circuit is predominantly reactive, and its value is not constant since it depends on the circuit geometry, which is essentially capable of being reshaped because the electrodes are vertically translatable, through several meters, during the period over which the arcs are established.
In order to enable the energy which is applied to the ferrous scrap in the furnace to be regulated in the desired manner, it is necessary to know the arc voltage at each of the electrodes on a continuous basis.
According to the prior art, the measurement of the arc voltage is derived from a calculation which starts with a measured voltage value, this measurement being made either at the terminals of the supply transformer or at the downline ends of the power supply cables which leave these terminals. The result of this measurement is very inaccurate, for the circuit impedance is variable, and its value cannot be determined at the moment at which the arc-voltage calculation is performed and, for this calculation, the impedance is assumed to have a constant mean value, an assumption which is necessarily inaccurate. In the case where the voltage is measured at the downline end of the supply cables, the measured voltage is incorrect due to the strong influence of the power currents, which induce error-voltages in the measuring circuit.